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Does the PSH bit force client to ack

asked 2019-05-20 18:09:11 +0000

Grrrshark gravatar image

Hi All,

First off, thank you to the awesome community. I have just started my packet analysis journey, and have become quite addicted.

My question is if the server/client sends 1 packet with the push bit set, would this cause the receiver to ack even though 2 packets were not sent? I have seen/read many times that the rule of TCP is ack every other packet, but occasionally in a capture I am looking at the sender will set the psh bit after sending only 1 packet, and the receiver will ack immediately (empty ack). What confuses me is the receiver will then immediately send another packet with data. Wouldn't the rule of TCP prevent an empty ack since only 1 packet was sent, and since the client is sending data, wouldn't it just piggy back off the first ack rather than send an empty ack, then send data?

Thank you, and please let me know if I am not making sense. I will attempt to clarify.

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answered 2019-05-20 18:23:04 +0000

SYN-bit gravatar image

The rule of TCP is there is no rule of TCP. There are howevers RFCs that a stack can follow or not. In the first RFC about TCP the ACK mechanism was defined as sending an ACK to every data packet received. Then in RFC 1122 it is proposed to only send an ACK every other data packet as an efficiency enhancement. So there is not one standard way to determine when to send an ACK as long as all data is being ACKed (within a reasonable timeframe).

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Thank you for the answer. So really the psh bit is send now (don't buffer), it does not solicit an immediate ack. Ack is fairly irrelevant as long as the data is being acked and TCP delta is staying reasonable?

Grrrshark gravatar imageGrrrshark ( 2019-05-20 18:31:01 +0000 )edit

That's how I understand it at least :-)

SYN-bit gravatar imageSYN-bit ( 2019-05-21 14:26:18 +0000 )edit

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Asked: 2019-05-20 18:09:11 +0000

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Last updated: May 20 '19